It is known that the enzymatic efficiency of both actomyosin ATPase and creatine phosphokinase (CPK) decline with advancing age. These are two enzymes that supply energy necessary for (cardiac) contraction. Consequently, this decrease in enzyme activity correlates well with the known decrements in cardiac function in older individuals. It is also known that if young animals are subjected to rigorous physical training (i.e., exercise), the levels of these important enzymes may be maintained throughout life, but if exercise is initiated at later ages, the response to training is altered. Infact, in old animals, there may be negative adaptation (decreased enzymatic activities following exercise) but the nature of this catabolic response is uncertain. Therefore, beginning at 1, 6, 12, or 18 months of age, male Fischer 344 rats will be subjected to an exercise regimen of 60 minutes swimming twice daily for six months; paired littermates will serve as sedentary controls. In the oldest age groups, there will be an additional experimental group that will be subjected to lower exercise intensity (30 minutes once a day). Animals from these several age groups will be sacrificed and the myocardial actomyosin isolated by standardized procedures utilizing differential ionic strength solubility. The ATPase activity will be estimated in terms of the rate of liberation of phosphorus from ATP under the appropriate ionic conditions of calcium and magnesium. CPK activity of myocardial homogenates prepared in dilute alkaline solution and purified by successive ammonium chloride and ethanol precipitations will be estimated by measuring the rate of liberation of creatine from phosphocreatine. This study should yield data indicating to what extent physical training at different periods of life may moderate the diminishing activity of these enzyme systems that occur during aging (as well as give information on the adaptive response that occurs during advanced age) in relation to the concomitant declining cardiac output in aging animals, including man, himself. If negative adaptation occurs at lower levels of exercise, such findings give credence to the idea that there is a threshold age beyond which the initiation of exercise may not be beneficial.